A permanent magnet type motor and a reluctance motor are driven by an inverter in systems such as electric trains and electric vehicles. In this case, a terminal voltage of the motor cannot exceed a maximum voltage the inverter can output. This necessitates execution of a flux weakening control so that the terminal voltage of the motor is not more than the maximum voltage during a constant power drive with high speed rotation. In the flux weakening control of the permanent magnet motor, an armature current is generally caused to flow in such a manner that magnetic flux generated by permanent magnet and magnetic flux generated by the armature current are directed opposite each other, that is, a negative d-axis current is generally caused to flow.
However, there is a possibility that control of the permanent magnet motor would become unstable when current for the flux weakening control is caused to fixedly flow in the direction of the d-axis in execution of the flux weakening control for an electric motor generating composite torque that is a combination of torque generated by permanent magnet (hereinafter, “magnet torque”) and reluctance torque. The reason for this is that the flux weakening control interferes with torque control that is executed to obtain a desired torque by control of current amplitude, with the result that effect of the flux weakening control cannot be achieved. Consequently, the motor terminal voltage cannot be limited to a range not more than a maximum voltage the inverter can output.
The aforementioned phenomenon is also likely to occur in an interior permanent magnet motor which includes permanent magnets embedded in a rotor core and outputs a composite torque that is a combination of magnet torque and reluctance torque. In view of the phenomenon, there has conventionally been provided a technique of carrying out the flux weakening control so that a constant torque curve changing in a curvilinear manner depending upon the depth or degree of flux weakening is followed.